1. Field of Invention
The invention concerns a magnetic disk substrate of plastic containing fibers.
2. Prior Art
U.S. Pat. No. 3,761,333 shows the use of substrates of plastic containing fibers for the production of magnetic disks. Such a substrate, together with magnetic foils, is placed in a mold and pressed to form a magnetic disk. In addition to the fact that this substrate has no substantially isotropic lamellar structure, it is not suitable for rotating at the high speeds required for present and future applications.
Extremely high storage densities necessitate an increasing number of revolutions. In addition, the problem of disk vibrations in the axial direction, i.e., in the direction parallel to the rotation axis of the disk, is becoming increasingly critical. In the case of rotating disks, there are two resonance frequencies, one of which, as a study by S. A. Tobias and R. N. Arnold "The Influence of Dynamical Imperfection on the vibration of Rotating Disks" PROC. I.M.E., Vol. 171, No. 2 (1957), 669-690, has shown, is particularly critical because at a given number of revolutions .OMEGA..sub.c, a slight static load is sufficient for resonance to occur. Such a slight static load may take the form of the magnetic head resting on or floating above the magnetic disk, so that the magnetic disk has to be operated below the critical number of revolutions.
For increasing the natural frequency of magnetic disks and for damping vibrations parallel to the rotation axis, both being necessary to obtain a higher number of revolutions, UK Pat. No. 1,264,716 shows mechanically reinforcing the magnetic disks to different degrees. For this purpose, disks with a diameter decreasing towards the outside are provided on one or both sides of the actual magnetic disk. In their centers, these additional disks are rigidly connected to the magnetic disk, being arranged on its outside. This leads to friction between the magnetic disk and the additional reinforcing disks when the magnetic disk is bent in the direction of the rotation axis. As a result of this friction, which occurs in particular at high speeds, i.e., high frequencies, the high-frequency vibrations are satisfactorily damped. Low-frequency vibrations, however, as occur in the vicinity of the above-mentioned critical number of revolutions, are very poorly damped. It can be assumed that the critical number of revolutions is reduced by subdividing the disks into several individual disks or by adding further disks with diameters decreasing towards the outside. Therefore, this known solution is equally unsuitable for producing magnetic disks or magnetic disk substrates, the number of revolutions of which can be considerably increased above the values presently obtainable.
U.S. Pat. No. 3,646,533 shows a magnetic disk structure, the core of which is structured in the form of a honeycomb and the outside of which is metallically coated. This honeycomb structure, in which the honeycombs extend in the direction of the rotation axis and which are subsequently sealed, for example, by copper plates, serves to damp mechanical vibrations parallel to the rotation axis, in order to obtain higher rotation speeds at a higher natural frequency of the disk core. An essential disadvantage is the thickness required for such a disk. Tests have shown that to obtain the same critical number of revolutions as for the presently employed AlMg5 magnetic disk substrates with a thickness of 3 mm, the "honeycomb" disk described above has to be thicker than 1 cm if copper disks of 0.3 to 0.5 mm thickness are used as outer surfaces. A greater disk thickness means, however, that there are fewer disks in a pack. A further disadvantage of this honeycomb structure is that when the structure is bent, unevenness results because of the sharp edges occurring. This leads to problems if an extremely high degree of evenness is required.